Marriage is a God ordained institution, not a Christian one.
Government is a God ordained institution, not a Christian one.
The government of Paul's day, Nero's government, one of the most wicked governments in history was ordained of God.
All of the Islamic governments of today are ordained of God. God put them in place. If we live in such a place, we are to submit to their ordinances, according to Romans 13. This is a New Testament command.
The marriage is a union between one man and one woman. It always was and always will be. "Jesus said concerning divorce "from the beginning it was not so." He said: "What God has joined together let no man put asunder." It is a union. But that union is performed by man, not God.
It can be performed by a justice of the peace.
It can be performed by a Muslim cleric.
It can be performed by a Sikh priest.
The union of one man and one woman was a command given to every man and woman on the earth, not just to Christians. Marriage is not an exclusive Christian institution. It is for all people. It was incorporated into the laws of our nation, and we aren't even a Christian nation. It is incorporated into the laws of other nations that are not even built on Christian principles. It is God's institution for all men and women, just as government is.
The fact that God will use some of the characteristics of a Godly marriage: its purity, sacrifice, etc. to compare it to Christ and the church, has no bearing on this subject. Marriage is for all people, not just for Christians. To say otherwise is ridiculous.